1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to a method of controlling data transmission mode in an orthogonal frequency division multiple access (OFDMA) wireless relay system.
2. Description of Related Art
A wireless relay system may reduce a transmission loss and improve a multiplex gain by transmitting data via a path passing through at least one relay station (RS), in addition to direct transmission of the data to a base station (BS) and a mobile station (MS). The wireless relay system has advantages in that a transmission loss, due to distance, may be reduced since transmission paths are divided into a plurality of relatively short paths, and an improved multiplex gain may be acquired since a Multiple-In Multiple-Out (MIMO) scheme for interoperation between a BS and an MS is utilized for a plurality of short path channels. However, the wireless relay system has a problem of a signal inter-symbol interference (ISI) between signals, the signals being inputted from multi paths, such as between the BS and the MS, between the MS and the RS, and between the RS and the MS.
To overcome the problem of the ISI, new systems which apply the wireless relay system to an OFDMA wireless relay system have been presented in a number of papers including I. Hammerstrom and A. Wittneben, “On the optimal power allocation for nonregenerative OFDM relay links,” in Proc. IEEE Int. Conf Commun., Istanbul, Turkey, Jun. 11-15, 2006, M. Herdin, “A chunk based OFDM amplify-and-forward relaying scheme for 4G mobile radio systems,” in Proc. IEEE Int. Conf Commun., Istanbul, Turkey, Jun. 11-15, 2006, and the like.
In the new systems presented in the above papers, by considering an amplifying-and-forward relay, under an assumption there is no direct communication between a BS and an MS, a power allocation method capable of maximizing a channel capacity for each subcarrier in the BS and an RS is provided, and an algorithm capable of maximizing the channel capacity by distinguishing a subcarrier mapping between the BS and the RS with a subcarrier mapping between the RS and the MS is provided.
Conventional art including the aforementioned systems presented in the above papers have a problem in that, the channel capacity is calculated by excluding a direct communication between the BS and the MS to simplify analysis. Also, a problem of a feedback overhead may exist, in that a transmitter is required to know a channel parameter in a mode subcarrier between the BS and the RS, and a channel parameter in a mode subcarrier between the RS and the MS for the power allocation in each of subcarriers. Also, when distinguishing the subcarrier mapping between the BS and the RS and between the RS and the MS, another problem may occur in that a mapping index is required to be transmitted for each of the subcarriers.
Accordingly, a new method of controlling a data transmission mode which can improve a cell capacity and expand a cell coverage by considering a direct transmission path from a BS to an MS, including a data path via an RS, is required.